1. Technical Field
The present invention relates to an electric power producing system using molten carbonate type fuel cell, and particularly relates to such a system whose differential pressure between cathode and anode chambers is made small.
2. Background Art
Fuel cell produces electricity and water at the same time through a chemical reaction of hydrogen of fuel and oxygen of air, which reaction is a reversal reaction of electrodialysis of water. Generally a fuel cell comprises an electrolyte plate, an air electrode (cathode electrode) and a fuel electrode (anode electrode), and the electrolyte plate is sandwiched between these two electrodes. As the fuel gas such as hydrogen is fed to the anode and the air containing carbon dioxide is fed to the cathode, the above-mentioned chemical reaction occurs to produce electric potential difference (or electric power) between the cathode and the anode. The power generation system also comprises a reformer which includes a reforming chamber and a combustion chamber. The fuel gas such as natural gas (NG) is reformed to a hydrogen-rich gas through the reformer. The fuel gas reacts with steam in the reforming chamber to be reformed to hydrogen gas and carbon monoxide gas. The reforming chamber is heated by heat form the combustion chamber in which fuel gas and air undergo combustion.
Referring to FIG. 2 of the accompanying drawings, which illustrates a conventional power generation system using molten carbonate type fuel cells, numeral 1 denotes the fuel cell, 2 denotes the anode chamber, 3 denotes the cathode chamber, 4 denotes the reformer, 5 denotes the reforming chamber and 6 the combustion chamber.
The fuel gas 7 such as NG is preheated by a fuel preheater 8 and desulfurized by a desulfurizer 9. Then, the fuel gas 7 is led into an ejector 10 and further into the reforming chamber 5 of the reformer 4 with steam 11. Water is changed to the steam 11 through an evaporator (preheater) 12 and a superheater 13 and introduced to the ejector 10. Then, the steam 11 goes to the reforming chamber 5 of the reformer 4 with the fuel gas 7, in which the fuel gas 7 and the steam 11 are reformed to hydrogen-rich gas, and then introduced into the anode chamber 2 of the fuel cell 1. Gases from the anode chamber 2 (called "anode exhaust gas") is about 700 degrees C (.degree.C.) in temperature and contains non-reacted hydrogen. Therefore, the condensate is separated from the anode exhaust gas by a separator 18 via a first heat exchanger 14, a fuel preheater 8, a second heat exchanger 15, a heater 16 and a condenser 17. After that, the anode exhaust gas is fed into the combustion chamber 6 of the reformer 4, as the fuel, via the second heat exchanger 15 and the first heat exchanger 17 by a blower 19. The temperature of the anode exhaust gas fed into the combustion chamber 6 is about 500 degrees C.
Air 20 is fed into an air preheater 22 by the blower 21 and preheated by part of gases discharged from the cathode chamber 3. Then, part of the air 20 is fed into the cathode chamber 3 whereas the remainder is fed into the combustion chamber 6 of the reformer 4. Non-reacted hydrogen contained in the anode exhaust gas is combusted in the combustion chamber 6 and combustion heat thereupon helps maintain the reforming reaction of the fuel gas 7 with the steam 11 in the reforming chamber 5. Combustion flue gas such as carbon dioxide is supplied to the cathode chamber 3.
Fuel used in the reformer 4 is the anode exhaust gas which is discharged from the anode chamber 2 and contains hydrogen. Entire hydrogen is not consumed in the anode chamber 2. This fuel gas is condensed in the condenser 17 and separated from water in the separator 18 before going to the combustion chamber 6 of the reformer 4. The air 20 which is preheated by the air preheater 22 and fed into the combustion chamber 6 is used in combustion of hydrogen contained in the anode exhaust gas. This combustion maintains the reaction temperature in the reforming chamber 5 of the reformer 4 at about 750 degrees C.
In the above-described conventional power generation system using fuel cell, however, the electrolyte migration and depletion may occur when the pressure difference between the anode and cathode chambers raises over a certain value since the electrolyte of the fuel cell is the molten carbonate. If the electrolyte depletion occurs, the power generation is no longer expected. In order to overcome this problem, or in order to maintain the pressure difference within a decent range, the pressure difference between anode gas and cathode gas have to be controlled. However, it is difficult to control this pressure difference since the anode exhaust gas is introduced to the combustion chamber of the reformer via several devices such as a heat exchanger and the cathode exhaust gas is also discharged through devices such as a heat exchanger.